SQUELCH CONTROL CIRCUITS

TM 11-5820-670-30

1-60. RADIO SET AN/ARC-131 INTERUNIT CIRCUIT DETAILS. (CONT)

See FO-17 while reading this section.

Squelch Disable Mode (DIS)

When SQUELCH control S9106 is set to DIS, the ground at contact 7 is connected through S9106 from

contact 5 and then to pin V of control unit connector J9101. The ground is then connected through

interconnecting wiring to pin 12 of receiver-transmitter connector P9601.

The ground at pin 12 (squelch disable ground) is connected to pin 25 of audiofrequency amplifier

connector P5501, and then through contacts A3 and A2 of relay K5501 to the coil of relay K5502. The

ground is used to energize relay K5502 which, in turn, couples +16 vdc through contacts A1 and A2 to

activate the output stage of the receiver audio amplifier.

Relay K5502 remains energized and the audio stages remain activated as long as the SQUELCH control

is in DIS position, and all audio signals are passed and amplified by the receiver-audio amplifier. The

audio signals are then coupled through pins 2 and 8 of connector P5501 and pins 13 and 14 of

receiver-transmitter connector P9601 to the control unit.

Carrier Squelch Mode (CARR)

When SQUELCH control S9106 is set to CARR, the ground at contact 7 is disconnected from contact 5,

and the ground applied at pin V of control unit connector J9101 is removed. Removal of the ground at

pin V causes relay K5502 in the audiofrequency amplifier to reenergize. This action removes +16 vdc

coupled to the audio output stages through contacts Al and A2 of relay K5502 and turns off the receiver

audio amplifier.

The noise, which is of significant amplitude in the absence of a received carrier signal, is coupled

from the receiver-audio amplifier through contacts 63 and 62 of relay K5501 to ac amplifier Q5201.

The noise is amplified by Q5201 and applied to dc amplifiers Q5203 and Q5204. Emitter follower Q5202

provides proper operating bias for ac amplifier Q5201. A portion of the amplified output dc amplifier

Q5204 is applied to the squelch filter.

With the SQUELCH control set to CARR, relays K5503 and K5504 are deenergized and the squelch filter

circuit is connected to attenuate the 7300 Hz noise components. All other frequencies are fed back to

Q5202 through the squelch filter. In this condition, the amplifiers are completely degenerative for all

frequencies except 7300 Hz and provide maximum gain at 7300 Hz.

The amplified noise components are coupled from dc amplifier Q5204 to noise rectifier CR5201 which

provides a filtered output to dc amplifier Q5205.

When receiver noise alone is present, the rectified noise signal from CR5201 causes Q5205 to con-

duct. When Q5205 conducts, the collector voltage applied through contacts 63 and 62 of relay K5505

to dc amplifier Q5207 causes Q5207 to turn off. Since S5207 controls the energizing current for relay

K5502, the relay is kept from energizing and +16 vdc is not applied to the output stage of the receiver-

audio amplifier, and the audio signals are prevented from passing.

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